Radiative forcing by contrails

A parametric study of the instantaneous radiative impact of contrails is pr esented using three different radiative transfer models for a series of mod el atmospheres and cloud parameters. Contrails are treated as geometrically and optically thin plane parallel homogeneous cirrus layers in a static at mosphere. The ice water content is varied as a function of ambient temperat ure. The model atmospheres include tropical, mid-latitude., and subarctic s ummer and winter atmospheres. Optically thin contrails cause a positive net forcing at top of the atmosphere. At the surface the radiative forcing is negative during daytime. The forcing increases with the optical depth and t he amount of contrail cover. At the top of the atmosphere, a mean contrail covets of 0.1% with average optical depth of 0.2 to 0.5 causes about 0.01 t o 0.03 Wm(-2) daily mean instantaneous radiative forcing. Contrails cool th e surface during the day and heat the surface during the night, and hence r educe the daily temperature amplitude. The net effect depends strongly on t he daily variation of contrail cloud cover. The indirect radiative forcing due to particle changes in natural cirrus clouds may be of the same magnitu de as the direct one due to additional cover.